LM3555

SNVS594E – JANUARY 2009 – REVISED NOVEMBER 2011

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In all cases during startup, the diode current will first ramp to 80 mA and then proceed to the target current. If the

torch/assist current is set to 60 mA, the LM3555 will first reach 80 mA and then drop to 60 mA.

The number of LEDs present in the system is recorded in a read only Diode Number (DN) bit of the fault register

(address 0x05). In Simple Mode, the number of LEDs present are output on the Strobe pin ('0' = 1 LED, '1' = 2

LEDs).

SINGLE LED OPERATION

In single LED operation, the LED flash current is allowed to reach the maximum level of 500 mA. By default, the

assist/torch current is set to 80 mA,and the flash current is set to 500 mA.

drops, the LM3555 will first transition from pass-mode to the minimum duty-cycle boost mode. In this mode, the

boost mode to standard boost mode. Standard boost mode adjusts the converters duty-cycle to maintain 300 mV

across the LM3555's current source.

Once the LM3555 transitions from pass mode to either boost mode, the part will stay in one of the boost modes

until the part is disabled or timed-out and then restarted.

DUAL LED OPERATION

In dual LED operation, the LED flash current is allowed to reach a maximum level of 400 mA. By default, the

assist/torch current is set to 60 mA, and the flash current is set to 320 mA.

During Dual LED operation, the output voltage will always be greater than the input voltage (assuming standard

white flash LEDs are used), forcing the LM3555 to be in boost mode over the entire input voltage range.

TORCH or ASSIST (CONTINUOUS CURRENT) OPERATION

There are two different continuous current modes on the LM3555: Torch and Assist.

enabled and disabled through by setting the value of the TEN bit in the Control Register (Address 0x04). TEN =

'1' allows an external Torch while TEN = '0' does not.

control mode, Assist mode is enabled by setting the Output Mode bits (OM1 and OM0) to '10' and setting the

the OEM bit is set to '0' or until a flash event occurs.

The LM3555 can drive one or two LEDs at continuous current levels ranging from 60 mA to 160 mA in 20 mA

steps. In Simple Control mode, the torch and assist current levels are equal to 60 mA for two LEDs or 80 mA for

FLASH (PULSED CURRENT) OPERATION

By driving both EN1 and EN2 high ('1') in Simple Mode, the part will enter flash mode and remain there until the

control pins are driven low '0', or a timeout event occurs. In Simple Mode, the flash current is equal to 500 mA

when driving a single LED and 320 mA when two LEDs are present. The default timeout duration is 850 ms.

will remain active as long as the OEN bit is set to a '1', and will terminate upon a timeout event. The safety timer

duration can be set in 50 ms intervals ranging from 100 ms to 850 ms by writing the desired value to the FT3-

FT0 bits in the Indicator and Timer Register (Address 0x02).

The Strobe pin provides added system flexibility in that it allows an additional external device (Camera Module,

bit and the Output Enable (OEN) bits in the Control Register (Address 0x04) must first be set to 1's.

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